1. The Field of the Invention
This invention relates to battery separators and particularly to battery separators carrying a plurality of polymeric ribs on at least one surface of the separator.
2. Description of the Prior Art
Battery separators are known to the art and their desired combination of performance characteristics are well defined. Essentially, a separator must have low electrical resistance, must be wettable by, but resistant to, the electrolytic fluid and must provide sufficient porosity to permit effective passage of electrolyte through the separator when positioned between adjacent positive and negative plates of the battery. Ideally, battery separators should provide minimal thickness and weight per unit area but at the same time have sufficient mechanical strength and integrity to sustain the conditions involved in the production and handling of the separator as well as the conditions encountered in the assembly and operation of the battery. Known separators are fabricated of separator base materials which include among others, cellulosic fibers, polyoiefin fibers, glass fibers, polyester fibers, phenol formaldhyde coated fibers and non-fiberous elastomeric polymers such as natural and synthetic elastomers.
Many known battery separators include protrusions carried by at least one surface of the separator. These protrusions may be carried in the form of embossed areas or as a plurality of solid ribs. Details relating to battery separators carrying such protrusions may be found in U.S. Pat. Nos. 2,465,493; 3,351,495; 3,725,130; 3,798,294; 4,000,352; 4,403,024; Canadian Pat. No. 518,249; U.K. Pat. No. 783,729 and German Pat. No. 26 84 06.
The application of protrusions to a separator surface is designed to improve the mechanical strength and integrity of the separator and/or to provide and maintain a space of predetermined thickness between the plate and battery separator. In general, either embossed or rib-like protrusions can suitably perform this function. Embossed protrusions however, can imply the possibility of non-uniform areas of thickness of the separator particularly in the embossed areas where some stretching of base material can occur. In practice, rib-like protrusions are preferred and these can be conveniently provided by extruding molten polymeric material onto the surface of the separator. The final separator usually comprises a plurality of parallel ribs which extend in continuous fashion across the surface of the separator. Details relating to a process for forming a plurality of polymeric ribs on a separator surface are described in U.S. Pat. No. 3,773,590 and in commonly owned, copending U.S. Patent Application Ser. No. 447,270, filed Dec. 10, 1982.
Continuous processes for providing permanent, extruded ribs on battery separator surfaces present special considerations and complicated problems. Initially, it is important that effective adhesion between the rib and separator be achieved almost immediately after extrusion of the rib material onto the separator surface. Normally, achievement of this adhesion is promoted by heating the separator material and extruding the polymeric material at relatively high temperatures to provide enough tackiness for the extruded rib to stick to the heated separator surface. However, close control must be maintained over the temperature of the extruded material to assure that the extruded rib will provide the predetermined height dimension when solidified. If the extrusion temperature is too high, the viscosity of the extruded material may permit excessive flow of the material before solidification. Excessive flow can result in decreased rib height and more importantly can result in increased rib width or thickness.
An important consideration in preparing polymeric rib carrying separators involves the thickness or width of the rib base. As used here, rib base thickness or width means the thickness or width of the contact area of the rib base with the separator surface. Ideally, the rib base width or thickness should be maintained at a minimum because the surface area of the separator covered by the rib base is not available for electrolyte transfer. Accordingly, increase in rib thickness - particularly at the rib base - decreases the effective electrolyte transfer surface area which in turn, increases the ohmic resistance of the separator.
One attempt to control rib base thickness has involved the extrusion of ribs having a substantially circular cross-section in order to minimize the contact area between the rib base and separator surface. However, this attempt has not proven sufficiently reliable in providing uniformly effective adhesion of the circular cross-section ribs to the separator surface. The above-mentioned U.S. Patent 3,773,590 teaches the extrusion of ribs which have substantially circular cross-section. However the rib carrying separator is passed between rollers while the ribs are still deformable and the ribs are compressed into the separator surface. This compression provides ribs of a rectangular or semi-rectangular cross-section and increases the rib base thickness of the extruded rib.
Another practice in the art involves extruding ribs of rectangular or trapezoidal cross-sections. A balance is achieved between the desired degree of tackiness for bonding and the temperature and viscosity of the extruded material by regulating the height of the molten rib material continually deposited on the moving surface of the separator. The height of the deposited molten rib material is usually somewhat greater than the height desired for the final solidified rib. After the molten material is deposited and bonded, the material is allowed to cool. Some decrease in height can occur on cooling because of shrinkage or settling and the rib base thickness usually increases until the bonded, molten material solidifies. Obviously, rib base thickness considerations become more complicated under this practice as the height of the rib is increased and ideally, the height of the rib should exceed the rib base thickness.
From the above description, it should be apparent that rib base height and thickness are primarily subject to the temperature and viscosity of the extruded material. However, additional factors are involved such as rate of extrusion and rate of separator sheet material travel. All of these are important and each must be carefully controlled and synchronized to achieve uniform rib height and minimized rib base thickness. Accordingly, there is an outstanding need in the art for a process for effectively bonding a plurality of ribs having uniform height and minimized rib base thickness to a separator sheet material. This invention is addressed in part to that outstanding need and provides a novel, relatively simple and inexpensive but particularly effective solution for the problem.
There is also another important art recognized need involved in preparing polymeric rib carrying battery separators. As those in the art know, various polymeric rib providing materials - particularly polyolefins - undergo severe shrinkage with cooling during solidification. In turn, this shrinkage can cause severe bowing or curling which adversely affects the flatness of the separator. U.S. Patent Application Ser. No. 447,270 mentioned before, provides an effective means for controlling the shrinkage problem by compressing portions along the length of the ribs to provide ribs having alternating elevated and depressed segments. The practice of the present invention also provides polymeric rib carrying battery separators which are substantially flat even though the polymeric rib providing material involves polyolefins. Accordingly, the present invention provides the capability for producing substantially flat rib carrying separators combined with the capability for effectively providing ribs of substantially uniform height and cross-sectional thickness as well as controlled rib base thickness or width.